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indented note block in fix_num_diff.rst

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2020-03-04 19:48:12 -07:00
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@ -49,29 +49,29 @@ by two times *Delta*.
.. note::
It is important to choose a suitable value for delta, the magnitude of
atom displacements that are used to generate finite difference
approximations to the exact forces. For typical systems, a value in
the range 1e-xxx to 1e-yyy will probably work well. However, the
best value will depend on a multitude of factors including the LAMMPS
unit style, the stiffness of the interatomic potential,the
thermodynamic state of the material being probed, and so on. The only
way to be sure that you have made a good choice is to do a
sensitivity study on a representative atomic configuration, sweeping
over a wide range of values of delta. If delta is too small, the
output forces will vary erratically due to truncation effects. If
delta is increased beyond a certain point, the output forces will
start to vary smoothly with delta, due to growing contributions from
higher order derivatives. In between these two limits, the numerical
force values should be largely independent of delta.
It is important to choose a suitable value for delta, the magnitude of
atom displacements that are used to generate finite difference
approximations to the exact forces. For typical systems, a value in
the range 1e-xxx to 1e-yyy will probably work well. However, the
best value will depend on a multitude of factors including the LAMMPS
unit style, the stiffness of the interatomic potential,the
thermodynamic state of the material being probed, and so on. The only
way to be sure that you have made a good choice is to do a
sensitivity study on a representative atomic configuration, sweeping
over a wide range of values of delta. If delta is too small, the
output forces will vary erratically due to truncation effects. If
delta is increased beyond a certain point, the output forces will
start to vary smoothly with delta, due to growing contributions from
higher order derivatives. In between these two limits, the numerical
force values should be largely independent of delta.
.. note::
The cost of each energy evaluation is essentially the cost of an MD
timestep. This invoking this fix once has a cost of 2N timesteps,
where N is the total number of atoms in the system (assuming all atoms
are included in the group). So this fix can be very expensive to use
for large systems.
The cost of each energy evaluation is essentially the cost of an MD
timestep. This invoking this fix once has a cost of 2N timesteps,
where N is the total number of atoms in the system (assuming all atoms
are included in the group). So this fix can be very expensive to use
for large systems.
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